The present invention relates to receiving systems for digital data which are transmitted after linear modulation over a time-varying channel with limited passband. Such a channel introduces distortions of its output signal relative to its input signal constituted by the data sequence transmitted with a symbol period T.
Generally, such receiving systems are predominantly comprised of a filter circuit for the output signal of the channel and a subsequent decision circuit for estimating and recovering each of the transmitted digital signals at the input of the channel on the basis of the output signal of the filter circuit. These estimations must occur in the rhythm of a clock whose frequency and phase are produced by a clock recovery circuit permitting a decision which minimizes the error rate.
This problem of synchronization by means of clock frequency and phase recovery is of very great importance for the quality of the transmission of the data. To ensure that the receiving system restitutes the transmitted information at the input of the channel as correctly as possible, it be indispensable that this system is capable of detecting the most significant instants of the output signal of the channel; nowadays this possibility is obtained by means of the above-mentioned recovered clock, a control of which at the receiving end enables a synchronization with the clock of the transmitter. However, the systems realized thus operate poorly when the transfer function of the channel varies with time.
Based on said last assumption, British Patent Specification No. 1,478,709 discloses a receiving system for data which, in the example considered, are transmitted by differential multiphase modulation. By means of correlation, starting from the line signal and the detected information, this system generates a signal representing the envelope of the signal elements used for transmission or, as a variant, the square of this envelope. In this Example the line signal is expressed by the relation: ##EQU1## where S.sub.1 (t) is the signal usually designated as elementary signal and resulting from the modulation of a carrier .omega..sub.c by the baseband signal g(t), so:
S.sub.1 (t)=g(t) cos .omega..sub.c t;
S.sub.2 (t) is the signal in quadrature-phase with S.sub.1 (t), so:
S.sub.2 (t)=g(t) sin .omega..sub.c t;
and where .rho..sub.k and .phi..sub.k are the discrete amplitude and phase values used to represent the data at the instants kT (where T is the symbol period of the data), said values having been taken from the set of the discrete values {.sub..rho.j } and {.phi..sub.j } used for transmission (the expression (1) indicates that the signal results from the super-position of a large number of signals corresponding to sequentially transmitted information elements, and thus expresses the penomenon known as intersymbol interferences). The envelope signal of the transmitted elementary signal is then defined by the expression: ##EQU2##
On the curve representative of the shape of the envelope signal thus defined, two (or more) points are defined at mutually equal distances from a predetermined time reference point and the information for controlling the clock on reception consists of the amplitude deviation between these two points of the curves (or in the event of more than two points, by the deviation in the position of the centre of gravity of these points relative to a reference position). The experiments and simulations performed show that the reception of the transmitted data seems optimum when this deviation becomes zero.
The system described in said British Patent Specification No. 1,478,709 has, however, very serious drawbacks. On the one hand, the principle adopted for cancelling the phase deviation permits of an accidental synchronization of the clock at an extremum of the curve (and not only at a maximum), which does not result in the desired optimalization. On the other hand, the system operates with the aid of feedback loops comprising adaptive equalizers and converges comparatively slowly. A further known fact is that the greatest disadvantage of such adaptive systems is, in general, their complicated character so that these systems are very difficult to employ at very high transmission rates. Finally, it should be noted that the absence of filtering at the output of the transmission channel is no guarantee that the proposed principle can be used, as the presence of real maximum is not certain.